Graphite and carbon materials, especially reinforced carbon-carbon composite substrate materials, are subject to many applications in modern industry, particularly in the aerospace and aviation fields. However, such materials, when unprotected, are subject to degradation at elevated temperatures. Since many applications involve high temperatures, resistance to high temperature and thermal shock are often required properties of the material.
Reinforced carbon-carbon composite substrates are generally constructed of fibers and bound by a carbon matrix, resulting in a material having excellent structural properties. Generally, carbonaceous fibers such as polyacrylonitrile, rayon or pitch-based fibers are utilized. Carbon-carbon impregnation materials generally are phenolic, furfuryl or pitch-based materials. Densification of the carbonaceous fibers can also be accomplished through use of chemical vapor deposition techniques to deposit carbon and form a matrix which strengthens the material. However, the use of a specific substrate material is not a limitation upon the present invention.
Graphite and carbon materials, including reinforced carbon-carbon composites, are subject to degradation, such as oxidation, when utilized in high temperature environments in the presence of oxygen. Generally, an unprotected graphite or carbon material will begin to oxidize at temperatures in excess of about 650.degree. F. in air. Therefore, in order to effectively utilize these materials in high temperature applications, it is necessary to provide protection from degradation, including oxidation. Copending applications Ser. No. 251,798now abandoned, and Ser. No. 252,117, now abandoned, disclose protective coatings for carbon, graphite and carbon-carbon composite materials. U.S. application Ser. No. 251,798, now abandoned, discloses a protective coating utilizing silicon, silicon carbide and alumina. U.S. application Ser. No. 252,117 now abandoned discloses a protective coating utilizing a composition containing boron, silicon carbide and silicon which is applied to carbon-carbon substrates. These types of primary protective coatings protect the carbon substrate very well at temperatures from about 2500.degree. to about 3000.degree. F. However, when the primary coating is subject to temperature cycles, it is theorized that the primary protective coatings can develop microcracks which become localized areas where oxidation can occur. Each time the substrate is cycled from ambient temperature to above about 2500.degree. F., the possibility of oxidation at the microcracks occurs which can progressively weaken the part after each cycle. Impregnation with tetraethyl orthosilicate (TEOS) which penetrates the microcracks in the primary coating and then, upon curing, converts to silicon dioxide, is known to provide protection to otherwise exposed carbonaceous surfaces. It has also been known to use a silicon carbide paste and liquid alkali silicate materials in combination with TEOS impregnation to provide fairly effective enhancement coatings, i.e., a coating placed on the silicon carbide primary coating.
One indication of high temperature degradation resistance is the percent weight change of the coated substrate or part that is exhibited over a period of exposure in an elevated temperature environment containing oxygen. It is desirable for a coated substrate not to exhibit a significant weight change after exposure to high temperatures in oxygen containing environments since this could affect the performance of the device incorporating such materials.
Accordingly, a need exists for a method and composition of matter for forming of sealant film over the protective coating on carbon substrates that provides improved resistance to degradation at elevated temperatures in the range of from about 650.degree. F. up to about 2500.degree. F. Further, a need exists for a method and composition of matter for a post treatment sealant for the protective coating on reinforced carbon-carbon composite substrates wherein protection from degradation is provided where frequent cycling occurs between -300.degree. F. and up to above 3500.degree. F. Thus, the present invention provides a post treatment sealant, which when utilized in combination with the primary protective coating, increases the dwell time at the temperature at which the carbon-carbon substrate is protected from oxidation. In an alternate embodiment, an enhancement coating is also provided and placed on the sealant coating.